Its actually fairly safe, considering the amount of shenanigans that drifters get upto on stretched tyres.
There are a few cases of the tyre coming off the bead, however from what I know these are few and far between.
It started out near as I can tell as a cost effective way of getting stiffer sidewalls from cheap tyres, and has progressed into the usual one-upmanship (in terms of skinny tyre on widest rim)